Crinkle Crags Excursion 7th September led by Clive Boulter
On the sunny morning of 7th September 20 members met at Three Shires Stone. On the long haul up to the Crinkles a couple of brief stops were made to prevent the party from being strung out too much and to catch breath! The first of these was at Red Tarn where small pieces of kidney and pencil ore mark small spoil tips and there is documentary evidence of workings in the 1600s and trials in the mid 1800s though suggestions of medieval activity exist. Half way up the steady climb to Long Top a fine example of welded ignimbrite introduced the topic of the day. Soon low cloud enveloped the party as we left the path and with visibility reduced to 20m it was essential to stay together.
Fortunately by the time we reached our first locality on the side of Adam-a-Cove the cloud had lifted and we could examine the point at which Hell was unleashed. Not wishing to enter the debate about distinguishing lava flows from peperitic sills the leader focused our attention on the pyroclastic rocks that mark the change from dominantly effusive behavior [Birker Fell Formation] to explosive caldera-forming eruptions; these started with a hundred or so metres of andesitic ignimbrite [Whorneyside Formation] before switching to rhyolitic composition [Airy’s Bridge Formation]. The lowermost few metres of the caldera fill has metric-scale alternations of welded and non-welded [cross-bedded] ignimbrite which were attributed to changes in the density of the pyroclastic currents. A diagonal walk towards Stonesty Pike allowed the magnitude of the pile of welded andesitic ignimbrite to be appreciated.
On Stonesty Pike a magnificent exposure of the Whorneyside andesitic bedded tuff revealed much about how it was formed and its subsequent eventful life. Typically around 30 metres thick, the tuff shows no signs of sedimentary structures but it is poorly sorted. This conundrum is resolved by observing the infills to erosional rills where the layers maintain their vertical thickness and hence prove an airfall origin. It appears that the andesite magma was intensely fragmented over a period of months when 20 to 30 km3 of water got into the magma chamber creating the largest phreatomagmatic eruption of all time. There is plenty of evidence of a moist eruption as there are many coated pellet [accretionary lapilli] layers which prove useful marker horizons for the thrust faults common in the exposure. These faults show that the removal of magma from the chamber was already leading to the breakup of the roof into variably tilted blocks bounded by volcano-tectonic faults. Centimetre-scale impact sags showed that some lapilli were sent on ballistic paths in the eruption.
Approximately along strike the angular unconformity between the darker, andesitic Whorneyside Tuff and the lighter, rhyolitic Stonesty Tuff was clearly seen. However this did not represent a large time gap but rather that a huge block of tuff had toppled, undergone erosion, and been covered by the Stonesty Tuff. It is difficult to estimate the time gap but in the context of a progressively evolving caldera these events took place in less time than it takes a thick ignimbrite to cool.
The next locality was a rather insignificant looking small exposure which marked the Isaac Gill Fault, a volcano-tectonic fault with more than 200 metres of offset. Abrupt rotation of bedding in the Whorneyside phreatoplinian tuff was seen a couple of metres from the slack occupied by the fault. The next locality provided key evidence for the origin of these faults during piecemeal caldera-collapse. In the rhyolitic welded ignimbrite the fiamme were seen to bend from their normal gentle dip continuously into a near vertical attitude towards a minor volcano-tectonic fault. These faults were clearly active whilst the ignimbrites were still hot and capable of ductile flow.
Our final location involved a climb to just below the summit to examine the Hanging Stone Tuff packed with multiply-rimmed pellets [accretionary lapilli], and many fragmented rims. Several similar horizons punctuate the rhyolitic pile of ignimbrites and prove a very useful means to identify individual units. Cross bedding in the tuff shows it formed form a pyroclastic current at the lower end of the density spectrum and marks small volumes of water getting into contact with the magma. When all the party had safely regained the path, still in good weather, an enthusiastic vote of thanks was given to our leader, Clive Boulter for a fascinating day on the high fells.
